An overview of Marathon Valley, seen from Opportunity on March 13, as the rover was positioned at the Valley’s northern side, along the western rim of Endeavour Crater. The scene spans from east, at left, to southeast. Image Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

With the coming of autumn in the northern hemisphere, the necessary preparations for the upcoming winter’s frosty weather are high in the to-do list of most people living across the US, Europe and Asia. Yet, preparing for Jack Frost is not a necessity that’s limited to Earthly occupants only, with NASA’s Opportunity rover which has been residing on Mars for the last 11 and a half years having already gone multiple times through a similar experience, as part of its mission of exploring another world. Located at the western rim of Endeavour Crater on a place called ‘Marathon Valley’ at the Red planet’s southern hemisphere, Opportunity is currently in the middle of local martian autumn with winter set to kick in the next couple of months. With the coming seasonal change the intrepid robotic explorer will be positioning itself to better withstand the cold days of the harsh martian winter (its seventh overall since it landed on the Red planet back in January 2004), all the while it conducts a detailed ‘walkabout’ survey of its surrounding terrain at Marathon Valley.

An orbital image of the 22-km-wide Endeavour Crater, which has been Opportunity’s home for the last four years, taken from NASA’s Mars Reconnaissance Orbiter. The yellow line indicates the path that has been traversed by Opportunity since its landing inside Eagle Crater in 2004 (upper left) until its arrival on Marathon Valley earlier this year. Image Credit: NASA /JPL-Caltech / MSSS / NMMNHS

Opportunity’s journey towards Marathon Valley and subsequent arrival there earlier this year has been a long time coming for the rover, one that had been started back in September 2008, after a series of observations from NASA’s Mars Reconnaissance Orbiter (MRO) which had determined that the 22-km-wide Endeavour Crater was home to phyllosilicate mineral deposits – a type of clay deposits which are formed in the presence of liquid water. Following a 3-year-long trek, Opportunity arrived at the western rim of Endeavour in August 2011 (which had been informally dubbed as a ‘second landing site’ for the mission), while beginning an arduous and challenging geologic investigation of the 300-meter-deep ancient crater that eventually helped scientists to confirm MRO’s orbital observations with ground-truth data and establishing that the entire site had been much warmer, wetter and habitable several billions of years ago, early in the planet’s history. Along the way, Opportunity ticked off several historic milestones, like reaching a total odometry of 40 km in July 2014 thus surpassing the previous off-world roving record that was previously held by the Soviet Union’s Lunokhod 2 lunar rover since 1973 and took the first historic images of comet C/2013 A1 Siding Spring as the latter made a close fly by of Mars from a distance of 139,500 kilometers in October 2014.

The coming of 2015 saw the rover passing several major milestones as well, with the mission celebrating in January its 11th anniversary of operations on martian soil, completing the first extraterrestrial marathon in March and passing the 4,000-sol mark of roving on the Red planet in late April and – an astonishing achievement considering that Opportunity was designed for just a 90-day mission! Following a brief 3-week hiatus in June when Opportunity had to cease most of its operations due to solar conjunction when Mars reached the opposite side of the Sun in its orbit as seen from Earth, the rover resumed its long trek and finally cruised into Marathon Valley in July, a 330-meter-wide depression at the northwestern rim of Endeavour Crater where the Mars Reconnaissance Orbiter has also spotted huge deposits of clay minerals possibly a bit different in nature than the ones that have been identified elsewhere across Endeavour Crater, indicating that the entire area is a treasure trove for scientists, in their study of Mars’ past and present habitability. “It’s great to be in Marathon Valley,” Dr. Steve Squyres, principal investigator for the Opportunity mission at the Cornell University in New York, told the Planetary Society. “It was a long haul. Based on the orbital data, the highest concentration of phyllosilicates we’ve ever seen with either rover is here. Just in terms of what we know about the place from orbit, it’s expected to be scientifically very productive.”

A collage of three raw images of Marathon Valley’ northern side (at left), taken by Opportunity’s navigation cameras on August 2. The distant floor of Endeavour Crater is clearly visible at the center. Image Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

Besides its scientific importance, Marathon Valley will also help Opportunity to receive as much sunlight as possible for its daily operations during the upcoming harsh winter. As the martian autumn progresses and gives way to winter, which for Mars’ southern hemisphere arrives in early January 2016, the Sun appears ever lower in the sky, limiting the amount of sunlight that Opportunity can gather with its solar panels for its daily power consumption. For this reason, the mission’s ground teams will command the rover beginning in October to drive towards the southern side of Marathon Valley which faces towards the Sun, thus helping it to maintain its power at the highest possible levels. “Our expectation is that Opportunity will be able to remain mobile through the winter,” says John Callas, project manager for the Opportunity mission at NASA’s Jet Propulsion Laboratory, in Pasadena, California. Opportunity’s re-stationing at the Marathon Valley’s sun-facing southern slope will not mean an end to science operations however. There are plenty of mineral deposits for the rover to investigate there as well during the long martian winter. “We have detective work to do in Marathon Valley for many months ahead,” explains Ray Arvidson, Deputy Principal Investigator for the mission, at the Washington University in St. Louis. “During the martian late fall and winter seasons Opportunity will conduct its measurements and traverses on the southern side of the valley. When spring arrives the rover will return to the valley floor for detailed measurements of outcrops that may host the clay minerals.”

While the last year has been a highly productive one for Opportunity it hasn’t been trouble-free, with the rover consistently facing issues with its onboard non-volatile flash memory which have affected its daily operations and overall science data storage abilities. Opportunity’s flash memory is a type of non-volatile memory that is used for safe storage of all of the data that are gathered throughout a sol’s activities, similar to the way a computer’s hard drive can permanently store information. Yet, starting in August 2014 Opportunity’s flash memory caused the rover to go through multiple reboots each day and causing the loss of engineering and science data, probably due to degradation resulting from its continuing operation for more than a decade. The mission’s engineering teams devised a plan to reformat the flash memory so that any ‘bad sectors’ could be identified and isolated, in the hopes that data storage would return back to normal. Even though a series of reformats in late 2014 and early 2015 originally seemed to have addressed the issue the computer resets unfortunately continued, albeit at a decreased rate, forcing engineers to entirely switch to the onboard volatile Random-Access Memory (RAM) for data storage.

A view of the northern edge of Marathon Valley, taken by Opportunity on Aug. 14, 2015. The image, which is a collage of six individual frames from Opportunity’s panoramic camera (Pancam), shows contrasting textures and colors at a place called Hinners Point (right) at the northern edge of Marathon Valley and swirling reddish zones on the valley floor to the left. Image Credit/Caption: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

By its nature, RAM memory can only hold any data temporarily as long as Opportunity’s systems are activated and turned on, which means that the rover must relay it to the orbiting Mars Odyssey spacecraft before it is deactivated at the end of each sol. Besides being more power-hungry, the use of RAM memory also slows down overall rover operations, but it doesn’t represent a limiting factor to the amount and quality of science that the rover can conduct. Despite these setbacks, which are still being investigated by engineers on the ground, mission planners are confident that Opportunity will be able to carry out most of its mission in the future as efficiently as it has been throughout its mission so far. “If we have to run the rest of the mission in RAM mode, we’ll run the rest of the mission in RAM mode,” says Squyres. “Opportunity can continue to accomplish science goals in this mode,” adds Callas. “Each day we transmit data that we collect that day. “It’s like a refrigerator that way. Without it, you couldn’t save any leftovers. Any food you prepare that day you would have to either eat or throw out. Without using flash memory, Opportunity needs to send home the high-priority data the same day it collects it, and lose any lower-priority data that can’t fit into the transmission.”

Having already surpassed its expected lifespan by more than 45 times, Opportunity is the rover that keeps on going, no matter what. With its major science objectives having been met right from the start of its mission more than 11 years ago, the intrepid robotic explorer continues to shed more light to the geologic history of Mars, as well as serving as a case in point for the longevity and durability of human artifacts on other worlds. In the end, scientific findings aside, the latter may well constitute Opportunity’s single greatest legacy.

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[…] the massive Endeavour crater, which is 14 miles (22 kilometers) across, examining rock outcrops in Marathon Valley. The rover has been on the rim since August 2011. Clay minerals have been identified in this region […]